In orthopaedics, stable fixation is a prerequisite for successful treatment of complex deformity, nonunion, and fracture. Bone plates are commonly used to obtain fixation and are commonly secured to the bone by screws. However, osteoporotic bone or bone with abnormally thinned cortices due to failed fixation or previous infection may not allow lasting screw purchase in the bone. Stability of the fractured bone gained by applying compression across the fracture site is quickly lost as the thin bone resorbs around the screws. This loss of compression is rapid, since loosening of the screws in the bone allows early toggling at the plate/screw interface.
As noted, most bone plates used in osteosynthesis are fixed solely by means of bone screws driven into the bone. Since the bone screws are only secured to bone, there is no rigid fixation of the screw to the bone plate. Therefore a loosening of the bone screws in the bone or a resorption of the bone can easily lead to a loosening of the bone plate.
It is known in plate osteosynthesis to use a nut on the cortex opposite a plate to fix the screw, and therefore the plate, to the bone. This method does not produce a direct fixation between bone screws and plate, but only compression of the bone located between the nut and the plate and penetrated by the screw.
In a variety of indications it is desirable to achieve a rigid fixation between bone screws and a bone plate in order to avoid subsequent loosening. For this purpose U.S. Pat. No. 5,053,036 teaches anchoring by frictional adhesion alone a bone screw with a specially designed conical head to a bone plate having corresponding conical holes. In this device both holes and screw heads must have a specific taper to obtain rigid fixation between them.
In U.S. Pat. No. 4,388,921, a bone screw with a specially designed conical or convex head is used in conjunction with a slotted insert having a matching tapered hole for the screw head. The special bone plate for use with this insert has cylindrical or spherical holes for the inserts. Because the insert is not threaded to receive the bone screw, fixation of the bone screw in the bone plate is dependent on the purchase of the bone screw threads in the bone.
U.S. Pat. No. 5,053,036 discloses a bone screw with a specially designed conical head used in conjunction with a slotted spherical insert in a bone plate. This design also requires the conforming tapers discussed above and in addition requires conforming spherical surfaces on the slotted sphere and in the bone plate.
Threaded conical inserts have been used to expand specially designed screw heads in order to lock the bone plate to the bone screw. Examples of this technique are found in U.S. Pat. Nos. 5,053,036 and 4,484,570. This design also requires conforming tapers on specially designed screws and plates.
U.S. Pat. No. 5,269,784 describes a nut interposed between the plate and the bone surface so that the plate is compressed between the head of the bone screw and the nut. An unthreaded bushing may be required in this arrangement between the screw head and the plate if the bone screw has an unthreaded portion near the head. Because the screw nut is installed on the underside of the bone plate, is must be introduced into the plate hole before or during operative placement of the bone plate. The nut must furthermore be retained in the plate by separate means during placement of the plate on the bone.